feat(ui): the capture context menu is always available (Ctrl+right-click); wordmark "ty" is YouTube red

1. The preview's right-click handler swallowed every right-click for the zoom box, so the
   element/backdrop context menu could not open at all. Sources and capture windows must be
   changeable AT ANY TIME — a creator swaps a window mid-stream without entering edit mode.
   Ctrl + right-click now bypasses the zoom path and lets WPF open the menu under the cursor;
   plain right-click keeps the zoom box. No menu markup changed.

2. The wordmark's "ty" is YouTube red (#ff0000), permanently. The token sweep had recoloured it
   to the interactive blue because it was a bare literal; DESIGN.md now records "never re-colour
   the wordmark" beside the token, and UiShellTests pins it.

Guard: UiShellTests gains the wordmark rule and the Ctrl+right-click chord.

Controls/TopBar.xaml

@@ -23,7 +23,9 @@
                     </ControlTemplate>
                 </Button.Template>
                 <TextBlock FontSize="22" FontWeight="Bold" VerticalAlignment="Center">
-                    <Run Text="LlamaCas" Foreground="{StaticResource Brush.OnSurface}"/><Run Text="ty" Foreground="{StaticResource Brush.Tertiary}"/><Run
+                    <Run Text="LlamaCas" Foreground="{StaticResource Brush.OnSurface}"/><!-- Creator ruling (2026-10-30, asked repeatedly): the wordmark's "ty" is YouTube red.
+                             It is brand, not state — do not re-tokenise it to an accent. -->
+                        <Run Text="ty" Foreground="{StaticResource Brush.Youtube}"/><Run
                         x:Name="BuildIdStamp" Text="{x:Static helpers:BuildStamp.Label}" FontSize="9"
                         BaselineAlignment="Superscript" Foreground="{StaticResource Brush.OnSurfaceVariant}"
                         ToolTipService.ToolTip="{x:Static helpers:BuildStamp.Label}"/><Run Text=" " FontSize="4"/>